1. Field of the Invention
This invention relates generally to induction motors, and, more particularly, to reversible single phase induction motors having overheating protection and the capability of starting without starting capacitors.
2. Description of the Related Art
In order to start a single phase induction motor, a quasi-second phase is needed. One method for providing the required phase shift is to couple an auxiliary winding from the single phase supply via a capacitor. The capacitor, which is commonly referred to as a starting capacitor in this context, increases the cost and size of the circuit and can sometimes decrease circuit reliability.
A method for providing a quasi-second phase without using a starting capacitor is embodied in a split phase motor in which an auxiliary winding with higher resistance than a primary run winding is used to create the phase shift. The resulting torque for a split phase motor is typically lower than desired.
Ward, U.S. Pat. No. 4,520,303, issued May 28, 1985, describes a self-starting single phase induction motor wherein the required quasi-second phase for starting the motor is provided by controlling the phase of the voltage applied to one stator winding of the motor relative to the phase of the voltage applied to another stator winding of the motor by means of an electronic switching arrangement and an associated control circuit.
Generating a phase shift by controlling relative phases of stator windings with the voltage sequences described in Ward results in a starting torque which can be too low for some motors. Furthermore it would be desirable to have a control circuit capable of performing the functions of overheating protection and motor speed control in addition to generating a quasi-second phase, especially if a universal circuit for overheating protection could be designed. Conventional overheating protection devices for protecting motors from thermal damage include a bimetallic element which bends in response to temperature levels and at a predetermined temperature level causes a switch to open. Because different types of motors have different temperature limitations, when conventional overheating protection devices are used, the different types of motors require distinct overheating protection devices with different bimetallic material compositions.